One known cleaning method for semiconductor devices such as silicon wafers comprises two steps, that are, a first step for removing organic and inorganic substances and a second step for removing foreign particulate matter adhering to the surface of a semiconductor device.
In the first step, organic and inorganic substances are oxidized, dissolved, and removed using a cleaning solution which typically contains sulfuric acid and hydrogen peroxide. The second step is directed to removal of foreign particles by separating them from the surface of a semiconductor device with a cleaning solution typically containing ammonium hydroxide and hydrogen peroxide. Through these two steps, the cleanliness of a semiconductor device can be improved. The removal of foreign particles in the second step is described in W. Kern, D. A. Poutinen, RCA Rev., 31, 187 (1970).
Strictly speaking, the above cleaning method composed of the two steps requires another two steps, namely, water cleaning processes that are respectively carried out subsequent to the first step and to the second step for washing each cleaning solution away by water. Further, if removal of residues left on the surface of a semiconductor device after etching of silicon and a silicon oxide film is necessary, there arises a need for cleaning the semiconductor device with the same solution as used in etching. The above prior art method disadvantageously involves many steps, so that it has a difficulty in improving throughput and requires a large-sized cleaning system.
One proposal to carry out cleaning through fewer steps is set out in Japanese Patent Laid-Open Publication No. 4-234118 (1992) and Japanese Patent Application No. 6-47297 (1994).
This cleaning method uses, as a cleaning solution, a mixture of a strong acid such as sulfuric acid, an oxidizer such as hydrogen peroxide and a fluorine-containing compound such as hydrofluoric acid. In this method, organic and inorganic substances, etching residues and foreign particulate matter can be removed by one step.
Specifically, this method is designed such that while organic and inorganic substances being oxidized and dissolved away by use of the strong acid, the surface of a semiconductor device is slightly etched by the fluorine-containing compound to establish a chemophobic surface so that etching residues and foreign particles can be removed.
It has been reported (see, for example, Kitahara, Watanabe, "Electrical Surface Phenomenon", Kyoritsu Shuppan, p. 300) that the extent to which foreign particles can be removed is determined by the following factors.
The potential energy of a foreign particle depending on the distance between the particle and the surface of the semiconductor device is determined, as shown in FIG. 1(a), by the balance between a repulsive force caused by a potential (.zeta.-potential) generated from electric charge on the particle surface and an attractive force caused by Van der Waals force.
If a semiconductor device and foreign particles are immersed in a solvent alkalified by aqueous ammonium hydroxide or the like, both semiconductor surface and particle surfaces are negatively charged under the influence of hydroxyl groups (OH.sup.-), increasing the repulsive force due to .zeta.-potential so that the potential energy is brought into the state shown in FIG. 1(b). As a result, the foreign particles become more likely to separate from the surface of the semiconductor device and unlikely to adhere to the surface of the semiconductor device again.
The above cleaning method provides easy removal of comparatively large foreign particles but has a difficulty in effectively removing minute foreign particles of about 0.1 .mu.m or less.
The reason for this is that, in minute foreign particles, the repulsive force caused by .zeta.-potential etc. is relatively small with the potential energy being as shown in FIG. 1(c), so that the foreign particles are likely to adhere to the surface of the semiconductor device again.
The reliable removal of minute foreign particles is becoming more and more important as the recent miniaturization of circuit patterns in semiconductor devices proceeds.
The present invention has been made in consideration of the above problems and one of the objects of the invention is therefore to provide a semiconductor device cleaning method capable of effectively removing minute foreign particles.